Wireless communication systems are well known in the art. Generally, such systems comprise communication stations, which transmit and receive wireless communication signals between each other. Depending upon the type of system, communication stations typically are one of two types: base stations or wireless transmit/receive units (WTRUs), which include mobile units.
The term WTRU as used herein includes, but is not limited to, a user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. WTRUs include personal communication devices, such as phones, video phones, and Internet ready phones that have network connections. In addition, WTRUs include portable personal computing devices, such as PDAs and notebook computers with wireless modems that have similar network capabilities. WTRUs that are portable or can otherwise change location are referred to as mobile units.
The term access point as used herein includes, but is not limited to, a base station, a Node B, a site controller, an access point, or other interfacing device in a wireless environment that provides WTRUs with wireless access to a network associated with the base station.
Typically, a network of base stations is provided where each base station is capable of conducting concurrent wireless communications with appropriately configured WTRUs. Some WTRUs are configured to conduct wireless communications directly between each other, i.e., without being relayed through a network via a base station. This is commonly called peer-to-peer wireless communications. WTRUs can be configured for use in multiple networks with both network and peer-to-peer communications capabilities.
One type of wireless system, called a wireless local area network (WLAN), can be configured to conduct wireless communications with WTRUs equipped with WLAN modems that are also able to conduct peer-to-peer communications with similarly equipped WTRUs. Currently, WLAN modems are being integrated into many traditional communicating and computing devices by manufacturers. For example, cellular phones, personal digital assistants, and laptop computers are being built with one or more WLAN modems.
A popular wireless local area network environment with one or more WLAN access points (APs) is built according to one of the IEEE 802.11 standards. The basic service set (BSS) is the building block of an IEEE 802.11 LAN and consists of WTRUs referred to as stations. The set of stations which can talk to each other can form a BSS. Multiple BSSs are interconnected through an architectural component, called a distribution system (DS), to form an extended service set (ESS). An access point (AP) is a station that provides access to the DS by providing DS services and generally allows concurrent access to the DS by multiple stations.
The 802.11 standards allow multiple transmission rates (and dynamic switching between rates) to be used to optimize throughput. Lower transmission rates have more robust modulation characteristics that allow greater range and/or better operation in noisy environments than higher transmission rates, which provide better throughput. It is an optimization challenge to always select the best (highest) possible rate for any given coverage and interference condition.
The currently specified transmission rates of various versions of the 802.11 standard are as follows:
StandardSupported Rates (Mbps)802.11 (original)1, 2802.11a6, 9, 12, 18, 24, 36, 48, 54802.11b1, 2, 5.5, 11802.11g1, 2, 5.5, 6, 9, 11, 12, 18, 24, 36, 48, 54
Conventionally, each 802.11 device has a rate control algorithm implemented in it that is controlled solely by that device. Specifically, uplink (UL) rate control is performed in stations and downlink (DL) rate control is performed in APs.
The algorithm for rate switching is not specified by the standards. It is left up to the station and AP implementation. Such rate control algorithms are usually proprietary and thus public information about them is limited. However, several algorithms have been described in academic and industry literature. Generally, they are relatively simple algorithms based on detecting missing acknowledgements (ACKs) and other statistics.
The 802.11 standard specifies a common medium access control (MAC) layer, which provides a variety of functions that support the operation of 802.11-based wireless LANs. In general, the MAC layer manages and maintains communications between stations and APs by coordinating access to a shared radio channel and utilizing protocols that enhance communications over a wireless medium. The MAC layer uses a physical (PHY) layer, such as defined in 802.11b or 802.11a, to perform the tasks of carrier sensing, transmission, and receiving of data frames.
In general, every transmitted MAC layer data frame is ACKed by the receiver. This is classically referred to as a “stop and wait” automatic repeat request (ARQ) protocol. If an ACK is not received by the transmitter (lost or never sent), then the original data frame is considered lost and the transmitter will go through the contention process again and try to re-send the data frame. A missing ACK assumes that the receiver does not get the ACK at all. However, a check to determine if an ACK frame can be partially missing (e.g., the CRC of the payload is bad, but the header information is intact) can be made. This can then be used in the decision process as a condition halfway between a missing ACK and a received ACK.
An example of an existing throughput-based rate control algorithm is as follows. First, 10% of the data is sent periodically at two data rates adjacent to the current data rate. Then, the throughput at each of the three different data rates is periodically evaluated by considering the amount of data that is successfully ACKed versus the amount of data transmitted at a given rate. Finally, a switchover is made to the data rate that provided the best throughput.
Such algorithms are one-dimensional in that they only consider their own link quality (via missing ACKs) during a given transmission. Unlike a typical station, APs generally have knowledge of the overall system and can accordingly consider more dimensions. For example, an AP can consider the UL data frame rate that was used by a given station within a given time window (e.g., the previous X seconds) as the starting point rate for its DL rate transmission to that station. An AP can also keep track of the last rate transmitted on the DL to a given station in the AP for a given period of time.
Sometimes it is better to transmit at higher rates to all stations (even with relatively high error rate) as transmitting at a low data rate to one user tends to slow down the entire system. The performance and switching points (e.g., how much error rate a typical station application can tolerate) of this type of system can be characterized and used in the AP rate control.
A cell-based finite state machine (FSM) type approach (as per 3GPP time division duplex (TDD) radio resource management (RRM)) can also be applied, where rate control can take different actions for different cell states (loads). The cell state can be set, for example, by a congestion control algorithm.
In addition, a wireless link can suffer from a high frame error rate (FER) compared to a wired link. The high FER can be due to a high traffic load, which results in more collisions and a consequently high FER; a bad wireless link condition, which can be due to high interference, fading, or a user moving away from an AP; or other reasons.